1. Field of the Invention
This invention relates to an anodizing solution for anodic oxidation of magnesium or its alloys.
More specifically, the present invention relates to an anodizing solution which is capable of forming an anodic oxidation coating having superior properties, particularly with regard to corrosion resistance and abrasion resistance, onto the surface of magnesium or magnesium alloys.
2. Description of Prior Art
Among the practical metallic materials, magnesium and its alloys are lightest and are superior in the mechanical properties. However, since they are chemically active and inferior in corrosion resistance, their practical application has been limited. Therefore, various surface treatments have been heretofore proposed and practiced.
The known surface treatments of magnesium and magnesium alloys may be divided roughly into the two groups i.e., a chemical conversion process and an anodic oxidation process.
The chemical conversion process comprises the steps of immersing magnesium or its alloys in a treating solution containing chromate or manganate as a main component and thereby chemically forming a corrosion protection coating, and, for example, includes the processes described as Classes 1 to 4 and 7 in JIS (Japanese Industrial Standard)-H-8651 (1978). On the other hand, in the anodic oxidation process, magnesium or magnesium alloy is immersed in a treatment solution containing, for example, aluminate, fluoride and chromate, as main components and an oxide coating is electrochemically formed by applying an alternating current or a direct current. Examples of such an anodic oxidation process include the processes specified as Classes 5 and 6 in JIS-H-8651 (1978), the HAE process specified in MIL Standard (MIL-M45202B) and the Dow 17 process.
However, these conventional processes are all unsatisfactory and have some problems to be improved. For example, the chemical conversion process has been employed for the purpose of tentative corrosion protection or under coating and a long-period corrosion resistance can not be ensured. Further, the coating formed is considerably poor in abrasion resistance.
On the other hand, although among the anodic oxidation processes the HAE process and the Dow 17 process set forth above are relatively advantageous, most processes indispensably need a certain system for treating mist which is generated from the treatment bath during the anodic oxidation processing operation and a waste water treatment system, since the treatment solutions contain heavy metal(s), such as manganese or chromium. Such systems are costly and, thus, economically disadvantageous.